In traditional electrical welding processes an electric arc produced substantially continuously for a relatively long period of time generates heat adequate to accomplish the welding function. The substantially continuous long duration arc requires an electrical power supply capable of producing on a substantially continuous basis output power at an adequate voltage and current level. Such power supplies usually include a main power transformer the size and temperature capacity of which are principal factors limiting the magnitude of the output or welding signal produced thereby.
One power supply presently used for shielded metal arc welding (SMAW), a term that will be used herein equivalently with continuous welding to indicate all continuous type welding processes including, for example, arc-air gouging, is manufactured and sold by Miller Electric Company under the designation Gold Star 600SS. That power supply includes a three phase low reactance or constant potential transformer with electronic control circuitry capable of providing a controlled continuous constant current DC output welding current. The transformer has three primaries receiving three phase input power and three secondaries, which are coupled to a full converter thyristor bridge circuit to produce the output welding signal. The bridge circuit is phase controlled in response to a feed-back signal providing information of the secondary current and a reference signal that is selectively adjustable by the user according to the desired power level of the welding signal.
In the Miller power supply closed loop feed-back is employed for the phase modulation as a function of the feed-back information. Such closed loop feed-back provides a number of advantages including, for example, constant current output, immunity to fluctuations in line voltage or line voltage compensation, and the ability to dial a desired current output. Moreover, the controlled bridge is preferably a full converter thristor bridge that assures turn-off of the thyristors, regardless of the impedance at the output terminals. In this manner, the full converter bridge may act as an output contactor.
In the art of stud welding the magnitude of the welding current, i.e. the output power, of the power supply usually must be larger, often by a factor of two or more, than the maximum power capability of many conventional stick welding power supplies. However, stud welding is a relatively short duration process in which the power supply usually is producing a welding signal only a fraction of a second.
One prior technique used to produce adequate welding current and welding voltage for stud welding was a brute force method with very large transformers and dropping resistors employed to obtain correct welding current, but this technique uses energy inefficiently. A second approach has been to use a magnetic amplifier or combination magnetic amplifier and saturable reactor to control the current of the weld. Although this method more efficiently utilizes the electromagnetic energy to obtain current levels for stud welding, in most cases those current levels are too high for stick welding and the magnetic amplifier or saturable reactor would not provide adequate range control. Therefore, in the past it was impractical to provide a dual function welding power supply for both stick welding and stud welding.
In past stud welding power supplies very high current capacities were necessary to provide hot plunge capability, i.e. to plunge the stud to engage the base metal while continuing nearly short circuit current flow. It would be desirable to limit such short circuit current while maintaining hot plunge ability.
There is incompatibility between stick welding and stud welding power supplies; in the former only power is controlled, whereas in the latter power, time, reduced current start and hot plunge functions are desirably controlled and generally a higher power output is necessary. It would be desirable, however, to be able efficiently to combine in a single power supply the capabilities of performing a wide range of both stick welding and stud welding operations.